Textures for games are usually powers of 2. For example, 64x64, 128x128, 256x256, 512x512, etc. Common sense dictates the higher resolution the better, but certain textures do not scale very well, so better ask the Developer what size(s) the texture needs to be in. Also remember that in a typical scene, many textures will be used so you need to pay attention to what works well together.